Effects of background gas pressure on pure electron plasma dynamics in the electron diffusion gauge experiment

Abstract

A pure electron plasma is confined in a Malmberg-Penning trap and its confinement and stability properties are studied. Of particular interest are the effects that collisions between plasma electrons and background neutral gas atoms have on the plasma expansion and on the evolution of the m=1 diocotron mode. Essential features of the m=1 diocotron mode dynamics in the absence of electron-neutral collisions have been verified to behave as expected. The mode frequency, the resistive growth rates, and the frequency shift at nonlinearly large amplitudes are all in good agreement with predictions. When background neutral gas is injected, the evolution of the mode amplitude is found to be sensitive to the gas pressure down to pressures of 5×10−10 Torr, the lowest base pressure achieved in the EDG device. The evolution of the plasma density profile has also been monitored in order to examine the shape of the evolving density profile, and to measure the expansion rate. The density profile has been observed to expand radially while maintaining a thermal equilibrium profile shape, as has been predicted theoretically. The plasma expansion rate is affected by the background neutral gas pressure, but the measured expansion rate is generally faster than the expansion rate predicted by considering only electron-neutral collisions.